The present invention relates to a spark plug used for an internal combustion engine. Moreover, the present invention relates to a method of producing the spark plug.
A spark plug is used for igniting an internal combustion engine of a motor vehicle and the like. For increasing engine output and reducing fuel consumption, temperature in a combustion chamber of the engine is likely to increase. For improving ignitability, a discharge portion of the spark plug is likely to protrude into the combustion chamber of the engine. Such type of engine is more and more increased in number. Under the above circumstance, the discharge portion of the spark plug is exposed to high temperature, thus causing failures (attributable to spark) such as wear, breakage and the like of the ground electrode.
Furthermore, as part of maintenance-free measures of the automotive engine, recently, durability of the spark plug is required with no replacement, for consecutive vehicle drive not less than 160,000 km or not less than 240,000 km (cumulative). To meet this requirement, the spark plug has the following metal: The central electrode and/or the ground electrode is made of a material having high heat conductivity such as Cu, Cu alloy, and the like (having heat conductivity equivalent to that of the former two). The material (hereinafter referred to as xe2x80x9cCu core and the likexe2x80x9d) is coated with Ni alloy. The Cu core and the like and the Ni alloy coating contribute to reduction in temperature, to thereby secure durability of the central electrode and/or the ground electrode.
Forming the Cu core and the like in the ground electrode for improving durability, however, reduces the temperature of the ground electrode attributable to thermal conduction. Although durability is secured, the ground electrode will cause reduction in temperature at high engine speed. Moreover, such reduction in temperature is seen even at intermediate engine speed and at low engine speed. Contacting the ground electrode that is reduced in temperature, flame kernel (generated during spark plug discharge) is likely to be extinguished. In other words, ignitability is deteriorated.
Moreover, another method for improving the durability of the ground electrode is taken into account. More specifically, use of another material for the ground electrode, which material is higher in heat resistance (strength). Included in the another material is, for example, a super heat resisting alloy and the like. Use of such another material, however, involves increase in ordinary temperature resistance (strength), and thereby involves deterioration in plastic machinability (bendability). Therefore, when the ground electrode (made of the another material) is bent, for example, in such a manner that a side face of the ground electrode faces the central electrode, plastic machinability (bendability) of the ground electrode is of difficulty. The difficulty in plastic machinability (bendability) is responsible for reduction in productivity.
It is therefore an object of the present invention to provide a spark plug that is used for an internal combustion engine at high engine speed, and that is excellent in durability and ignitability.
It is another object of the present invention to provide a method of producing the above mentioned spark plug.
According to a first aspect of the present invention, there is provided a spark plug comprising: a central electrode; an insulator surrounding the central electrode radially; a metallic shell surrounding the insulator radially; and a ground electrode having a first end connected to the metallic shell and a second end defining a side face. The ground electrode is so bent that the side face of the second end faces the central electrode. The ground electrode contains: a nickel in a range from 58% to 71% by weight, a chromium in a range from 21% to 25% by weight, an iron in a range from 7% to 20% by weight, and an aluminum in a range from 1% to 2% by weight. The ground electrode has a Vickers hardness in a range from Hv 140 to Hv 220 measured through a Vickers hardness test specified in Japanese Industrial Standard Z2244. A load 9.8 N is applied to the ground electrode in the Vickers hardness test.
According to a second aspect of the present invention, there is provided a method of producing a spark plug having a central electrode; an insulator surrounding the central electrode radially; a metallic shell surrounding the insulator radially; and a ground electrode having a first end connected to the metallic shell and a second end defining a side face. The ground electrode is so bent that the side face of the second end faces the central electrode. The method comprises the following sequential operations of: preparing the ground electrode composed of an alloy material, annealing the alloy material of the ground electrode at an annealing temperature not lower than 800xc2x0 C., so as to allow the alloy material of the ground electrode to have a Vickers hardness in a range from Hv 140 to Hv 220 measured through a Vickers hardness test specified in Japanese Industrial Standard Z2244; welding the ground electrode to the metallic shell; and bending the ground electrode in such a manner as to allow the side face of the second end of the ground electrode to face the central electrode. The alloy material composing the ground electrode at the preparation contains: a nickel in a range from 58% to 71% by weight, a chromium in a range from 21% to 25% by weight, an iron in a range from 7% to 20% by weight, and an aluminum in a range from 1% to 2% by weight. A load 9.8 N is applied to the ground electrode in the Vickers hardness test.
A spark plug under the present invention has a ground electrode composed of an alloy containing Ni 58% to 71% by weight, Cr 21% to 25% by weight, Fe 7% to 20% by weight, and Al 1% to 2% by weight. Thereby, the ground electrode secures sufficient durability at high temperature. The thus obtained ground electrode is preferably used for a combustion chamber at high temperature caused by high engine speed of the internal combustion engine.
Moreover, conventionally, improvement in high temperature durability (namely, heat resistance, oxidation resistance, and the like) occasionally deteriorates plastic machinability (bendability) of the alloy. Vickers hardness (Hv 140 to Hv 220) of the ground electrode under the present invention, however, features a good plastic machinability (bendability). Therefore, even when the ground electrode is bent in such a manner that a side face of the ground electrode faces a central electrode, plastic machining (bending) of the ground electrode is easy. The easy plastic machining (bending) is expected to contribute to improvement in productivity.
Vickers hardness higher than Hv 220 makes the alloy (composing the ground electrode) too hard, to thereby make it unfavorably difficult to bend the ground electrode. Moreover, annealing is carried out for improving bendability. In this case, however, annealing the ground electrode to such an extent as higher than Hv 220 in hardness requires annealing condition for about 800xc2x0 C. This temperature causes deposition of carbide on the grain boundary, to thereby deteriorate toughness. As a result, the ground electrode may cause minor cracks and the like during bending operation. With the cracks, the electrode may cause an unfavorable breakage attributable to vibrations and the like caused when the spark plug is used.
Contrary to the above, obtaining Vickers hardness lower than Hv 140 requires the annealing temperature as high as 1150xc2x0 C. This temperature is responsible for remarkable grain growth, to thereby cause grain corrosion attributable to S, Pb and the like. As a result, the ground electrode is likely to be broken. Moreover, some of the after-mentioned methods of producing the spark plug are not capable of producing the ground electrode with ease.
The ground electrode is more preferably has Vickers hardness in a range from Hv 160 to Hv 200.
Obtaining the above Vickers hardness (Hv 140 to Hv 220) of the ground electrode requires annealing, at not lower than 800xc2x0 C., the alloy that contains the above elements (Ni 58% to 71% by weight, Cr 21% to 25% by weight, Fe 7% to 20% by weight, and Al 1% to 2% by weight). Heating and keeping the ground electrode at not lower than 800xc2x0 C. softens the alloy, to thereby allow the ground electrode to have Vickers hardness from Hv 140 to Hv 220. The thus obtained Vickers hardness is preferable for bending operation. Too high annealing temperature, however, may cause failures such as enlargement of the crystal grain, shedding (drop) and cracks. Therefore, the annealing temperature has an upper limit 1150xc2x0 C.
The annealing temperature higher than 1150xc2x0 C. excessively promotes the grain growth of the alloy composing the ground electrode, and thereby the alloy is likely to be broken.
Contrary to this, the annealing temperature lower than 800xc2x0 C. is not sufficient for annealing the alloy. Therefore, preferable hardness (Hv 140 to Hv 220) is not provided for the ground electrode. Especially, keeping at annealing temperature 700xc2x0 C. to 800xc2x0 C. for a long time causes unfavorable deposition of carbide on the grain boundary. Thereby, the alloy is likely to be brittle. Further brittleness of the alloy causes the bent portion (formed during bending operation of the ground electrode) of the ground electrode to assume minor cracks. To further control the deposition of the carbide on the grain boundary, the annealing temperature is preferably set at not lower than 850xc2x0 C.
For controlling formation of the carbide (responsible for brittle alloy), increased cooling speed at 700xc2x0 C. to 800xc2x0 C. is preferable. More specifically, the annealing should be carried out in the manner described in the following one sentence: An alloy wire or an alloy band (the two kinds of alloy are hereinafter referred to as alloy material), which is a material of the ground electrode, is fed into a cylindrical (or pipe) annealing furnace at a constant feed speed. In the above annealing manner, the alloy material soon after passing through the above cylindrical annealing furnace is cooled faster than the one through an ordinary annealing furnace. The above increased cooling speed contributes to control of the deposition of the carbide on the grain boundary. Furthermore, the control of the carbide deposition prevents embrittlement of the alloy, to thereby prevent breakage and the like of the ground electrode. Varying length of the cylindrical furnace or the feed speed of the alloy material adjusts the annealing (keeping) period, cooling speed and the like.
Under the present invention, the ground electrode is improved in durability, leaving no need for measures to improve corrosion resistance. As a result, good ignitability is secured. For example, the spark plug under the present invention is unlikely to need for embedment of Cu core and the like (that is used for improving durability) into the ground electrode.
A conventional ground electrode is occasionally broken due to heat history (thermal hysteresis) attributable to fluctuation in temperature in the combustion chamber, when the conventional ground electrode is used in the internal combustion engine that is operated frequently at high speed.
Contrary to the above, the ground electrode used for the spark plug under the present invention has the alloy that is excellent in heat resistance. Composing the ground electrode with the above alloy is effective for preventing failures such as breakage.
For preventing the ground electrode from breakage, the ground electrode is composed of the alloy containing the above elements (Ni 58% to 71% by weight, Cr 21% to 25% by weight, Fe 7% to 20% by weight, and Al 1% to 2% by weight).
In addition, adopting the spark plug having the ground electrode in the following constitution contributes to the prevention of the breakage of the ground electrode: The ground electrode forms a peak end area extending, in an axial direction of the ground electrode, from a predetermined intermediate position to a peak end of the ground electrode. In the above constitution, the ground electrode is more reduced in cross section in the axial direction toward the peak end.
In the specification, xe2x80x9cdimension of the axial cross sectionxe2x80x9d of the ground electrode is defined in the following manner: 1. Draw two parallel external tangents to an outline of the axial cross section. The two parallel external tangents should not run across an internal portion of the outline of the axial cross section. 2. Select the external tangents having the most distant spacing.
The other objects and features of the present invention will become understood from the following description with reference to the accompanying drawings.